Abstract
Active transport of sugars into bacteria occurs through symporters driven by ion gradients. LacY is the most well-studied proton sugar symporter, whereas vSGLT is the most characterized sodium sugar symporter. These are members of the major facilitator (MFS) and the amino acid-Polyamine organocation (APS) transporter superfamilies. While there is no structural homology between these transporters, they operate by a similar mechanism. They are nano-machines driven by their respective ion electrochemical potential gradients across the membrane. LacY has 12 transmembrane helices (TMs) organized in two 6-TM bundles, each containing two 3-helix TM repeats. vSGLT has a core structure of 10 TM helices organized in two inverted repeats (TM 1–5 and TM 6–10). In each case, a single sugar is bound in a central cavity and sugar selectivity is determined by hydrogen- and hydrophobic- bonding with side chains in the binding site. In vSGLT, the sodium-binding site is formed through coordination with carbonyl- and hydroxyl-oxygens from neighboring side chains, whereas in LacY the proton (H3O+) site is thought to be a single glutamate residue (Glu325). The remaining challenge for both transporters is to determine how ion electrochemical potential gradients drive uphill sugar transport.
Highlights
Cotransport was first proposed for animal cells by Bob Crane and shortly thereafter extended to bacteria by Peter Mitchel where the process was renamed symport [1,2]
lactose symporter (LacY) is capable of transporting galactose with a low affinity but is unable to transport glucose or glucosides
In the initial structure of vSGLT, it was found to have 14 irregular trans-membrane helices with galactose bound in a central occluded cavity (Figure 3)
Summary
Cotransport was first proposed for animal cells by Bob Crane and shortly thereafter extended to bacteria by Peter Mitchel where the process was renamed symport [1,2]. It is well established, that the energy for active transport of sugars, and other substrates, is provided by ion electrochemical potential gradients across the cell membrane, which are primarily protons in bacteria and sodium in animal cells. LacY and vSGLT to gain insight into their transport mechanisms
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